1. Field of the Invention
The present invention relates to the field of decreasing the inductance of a capacitor. More specifically, the present invention relates to decreasing the inductance of a capacitor, such as a bypass capacitor, mounted on the surface of an integrated circuit board.
2. Description of the Related Art
Computer systems are information handling electronic systems which can be designed to give independent computing power to one user or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, personal computers, internet terminals, and notebooks. Computer systems include desk top, floor standing, rack mounted, or portable versions. A typical computer system includes at least one system processor, associated memory and control logic, and a number of peripheral devices that provide input and output for the system. Such peripheral devices may include display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, CD-ROM drives, printers, network capability cards, terminal devices, modems, televisions, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives, CD-R drives, or DVDs.
Components of a computer system or other electronic system may be operably coupled to each other by computer busses or other types of signal lines. Computer busses typically include a plurality of signal lines which are current pathways for allowing electronic signals to propagate along the bus. The signal lines of a computer system can be implemented as signal traces either on a circuit board or embedded in a circuit board.
Computer systems are typically assembled using circuit boards on which integrated circuits have been mounted. It is also known to include a voltage plane and a reference voltage plane (sometimes referred to as a xe2x80x9cground planexe2x80x9d) in an integrated circuit board. It is also known to incorporate a via, or conductive path, which can be used to electrically couple surface mounted structures, such as capacitors, with a voltage plane or ground plane.
As computer bus speeds have increased, the impedance of the signal paths of the busses has become more significant in the operation of the bus and accordingly the electronic system. Transmission rates between a memory controller and memory models may reach as high 800 MHz. This high data transfer rate is achieved in part by requiring that the physical characteristics of the bus between the memory controller and memory modules be within certain parameters. For example, under a specification for a memory module licensed by RAMBUS INC(trademark) the impedance of the signal paths between the memory controller and a memory module is specified at 28 ohms.
Memory busses may utilize electrically conductive vias to route the signals among embedded signal layers in a circuit board and trace signal layers on the planar surface of the circuit board. A typical via extending through a 62 mil thick circuit board exhibits an impedance of about 40 to 60 ohms, depending on the via diameter, pad sizes, and the position of the signaling layers that the via electrically couples. The impedance of the via and the internal impedance of a surface mounted structure, such as a capacitor, are cumulative. Higher impedance may lower the quality of signal transmission of the memory bus due to signal/noise margin degradation.
It is known that impedance is related capacitance and inductance. The relationship is understood to be given in Equation 1. From Equation 1 it can be seen that increasing the capacitance decreases impedance. Increasing capacitance can be accomplished by adding capacitors in parallel. However increasing the number of capacitors requires additional area on the surface of an integrated circuit board and can increase cost to the manufacturer and consumer.                     Z        =                              1                          j              ⁢                              xe2x80x83                            ⁢              wC                                +                      j            ⁢                          xe2x80x83                        ⁢            wL                                              Equation        ⁢                  xe2x80x83                ⁢        1            
Where: Z represents impedance
C represents capacitance
L represents inductance
w represents frequency; and
j represents the imaginary part of the complex number
It is known that increasing the ground pad size increases capacitance represented by C in Equation 1. As shown in Equation 1 above, increasing C decreases impedance. Therefore increasing the ground pad size decreases impedance. It is also known that inductance is proportional to the length of the trace. The length of the trace can be represented by L in Equation 1 above. Therefore, from Equation 1 it can be seen that decreasing the trace length also decreases the total impedance.
U.S. patent application Ser. No. 09/395,788 filed on Sep. 14, 1999 titled xe2x80x9cCapacitive Structure for Via Impedance Tuningxe2x80x9d (referred to as the xe2x80x9c788 patentxe2x80x9d) naming inventors David A. Baranauskas and Douglas E. Wallace, Jr. teaches tuning the impedance of a via. However methods may be available which reduce the impedance more than the method taught by the ""788 patent. Also, methods of reducing the impedance of a capacitor may be developed which are less expensive and may be implemented in conjunction with the method taught in the ""788 patent.
Similarly, U.S. patent application Ser. No. 09/605,905 (the xe2x80x9c905 applicationxe2x80x9d) filed on Jun. 28, 2000 titled xe2x80x9cPrinted Circuit Assembly having Conductive Pad Array with In-Line Via Placementxe2x80x9d naming Doreen S. Fisher and Thad McMillian as inventors, inventors teaches a conductive pad array with an in-line via placement. However this disclosure teaches a placement of vias which provides additional space on the surface of the integrated circuit board. The ""905 application does not teach a means to reduce inductance of capacitors.
The problem of impedance is noticed in determining the number of capacitors necessary to couple a voltage plane and a ground plane. Typically, the voltage plane and ground plane are adjacent and parallel, separated only by the core plane. This configuration of the voltage plane to the ground plane provides a low impedance path for a returning signal. This low impedance path minimizes the noise at high frequency. When a high frequency current (sometimes called a displacement current) is induced from the voltage plane to the ground plane through a capacitor it is said the signal xe2x80x9cjumpsxe2x80x9d or xe2x80x9cbypassesxe2x80x9d the conductive path. The capacitor through which the displacement current flows is called a xe2x80x9cbypassxe2x80x9d capacitor.
FIG. 1 shows a cross-sectional view of a capacitor in an operational configuration on integrated circuit board 100. In the configuration shown in FIG. 1, the integrated circuit board includes the following; dielectric layer 170, voltage plane 140, core layer 160 and ground plane 150. Voltage plane 140 and ground plane 150 are parallel and are typically separated by core layer 160. In many computers, voltage plane 140 is maintained at a pre-determined voltage over ground plane 150. For example, a Dell(copyright) Dimension(trademark) computer manufactured and sold by Dell Computer Corporation of Austin, Tex. has a voltage plane normally operating at 3.3 volts above the ground plane.
Still referring to FIG. 1, capacitor 110 is electrically coupled to voltage pad 130 as shown. Voltage pad 130 is electrically coupled to voltage plane 140 by some means (not shown). Similarly, ground pad 120 is electrically coupled to trace 190. Trace 190 is coupled by some means (not shown) to ground plane 150.
In the prior art the Institute of Electrical and Electronic Engineers (IEEE) has adopted a standard configuration of a surface mounted capacitor. The standard configuration provides information to allow manufacturers and designers to provide uniform connections from a surface mounted capacitor to a voltage plane. For a single capacitor, it is known to have an equal number of connections from the voltage plane to the voltage pad and the ground plane to the ground pad.
Referring to FIG. 2, capacitor 210 is shown with voltage pad 220. Voltage pad 220 is shown electrically coupled to trace 230. Trace 230 is shown coupled to via 240. Typically, the trace is located on the surface of the integrated circuit board. The industry standard calls for a minimum 25 mil trace width and a corresponding trace length of 50 mils. (Note, a xe2x80x9cmilxe2x80x9d is a commonly used unit of measurement representing 0.001xe2x80x3.) Similarly, the industry standard configuration specifies 46 mils as the diameter of a via connecting the capacitor to a planar surface within the integrated circuit board. However, even in the configuration adopted by the IEEE the impedance of a capacitor is unacceptably high. High impedance is particularly noticeable for bypass capacitors at the operating frequencies of computers being currently manufactured.
What is needed is a means to decrease the impedance of capacitors mounted on integrated circuit boards. A means of decreasing impedance of board mounted capacitors operating over 100 MHZ would be particularly beneficial.
In accordance with the present disclosure, a method is taught of reducing the inductance of a capacitor. The inductance of the capacitor is reduced by connecting the capacitor directly to a via. In one embodiment inductance of a capacitor is reduced by a plurality of via, the number of via greater than the number of electrical couplings from the voltage pad to the voltage plane. Specifically, inductance of a capacitor having less than six via is reduced in a range from 400 MHz to 800 MHz. In one embodiment the capacitor has a ground pad of a minimum size. In another embodiment the capacitor is electrically coupled to a trace having a length reduced to minimize inductance.
The disclosure teaches a method of designing an integrated circuit board having a capacitor to reduce the inductance of the integrated circuit board. The disclosure also teaches a computer system manufactured with a circuit board including a capacitor having a reduced impedance.
The foregoing is a summary and this contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting.